1. Field of the Invention
The present invention relates to a drive mode selecting system for a four-wheel-drive motor vehicle having a drive system capable of driving a front wheel drive mechanism and a rear wheel drive mechanism, and more particularly to a drive mode selecting system for a four-wheel-drive motor vehicle having a drive system including a clutch for interconnecting front and rear wheel drive mechanisms substantially rigidly for transmitting relative torque between these front and rear wheel drive mechanisms.
2. Description of the Relevant Art
Japanese Laid-Open Utility Model Publication No. 59-188731 discloses a four-wheel-drive (4WD) motor vehicle in which front and rear wheels are coupled to each other substantially rigidly through a drive system having a viscous fluid coupling, the drive system also including a clutch disposed between the front and rear wheels and disengageable to cut off the transmission of drive power to the rear wheels when the motor vehicle is braked.
By thus cutting off the transmission of drive power to the rear wheels when the brake is applied, the distribution of braking forces to the front and rear wheels while the motor vehicle is running is prevented from being varied due to the limited differential capability of the viscous fluid coupling.
In some automatic transmission 4WD motor vehicles employing such a clutch, the front wheels are directly coupled to the engine, and drive power from the engine is transmitted through the clutch to the rear wheels. When the accelerator pedal is depressed while depressing the brake pedal, the brake is applied and the transmission of the drive power to the rear wheels is cut off. Therefore, the drive power is transmitted only to the front wheels, and the speed of rotation of the rear wheels is greatly lowered as compared with the front wheels as shown in FIG. 5 of the accompanying drawings. Since there is still large differential rotation between the front and rear wheels even after the brake is released, where the clutch comprises a dog clutch, the dog clutch cannot immediately be engaged due to such differential rotation.